The present invention deals with a process for the manufacture of cellulosic moulded bodies, in particular cellulosic flat films and cellulosic membranes in the form of flat membranes whereby a solution of cellulose in an aqueous tertiary amine oxide is moulded in film form by means of an extrusion die, which as an oblong extrusion gap, and led through an air gap into a precipitation bath whereby the cellulosic flat film is formed in the precipitation bath.
From U.S. Pat. No. 2,179,181 it is known that tertiary amine oxides have the ability to dissolve cellulose and that cellulosic moulded bodies such as fibers can be won from these solutions as a result of precipitation/regeneration. A process for the production of solutions of this kind is for example known from EP-A-0 356 419. According to this publication first of all a suspension of cellulose is prepared in an aqueous tertiary amine oxide. The amine oxide contains up to 40 weight % water. The aqueous cellulose suspension is transferred into the solution in a thin film treatment apparatus.
From DE-A-28 44 163 it is known for the production of cellulose fibers that an air gap is provided between the spinning nozzle and the precipitation bath to achieve drawing at the nozzle. This nozzle drawing is necessary since the stretching of the filaments is made more difficult after the contact of the moulded spinning solution with the aqueous precipitation bath. In the precipitation bath the fibre structure set in the air gap is fixed.
A process for the production of cellulosic threads is, furthermore, known from DE-A-28 30 685 whereby a solution of cellulose is formed to filaments in a tertiary amine oxide in a warm condition, the filaments are cooled down with air and finally introduced to a precipitation bath to precipitate the dissolved cellulose. The surface of the spun filaments is, furthermore, moistened with water to reduce their tendency to stick to neighboring filaments.
From DE-A-195 15 137 a process is known for the production of flat films whereby first of all a tubular film is formed using a ring nozzle, said film being cut to flat films following washing and drying. When manufacturing the tubular films, the extruded tube is extended in the air gap both in the drafting direction and in the transverse direction. This happens as a result of gas pressure effective in the inside of the tube. The disadvantages of this process lie in the complicated design of the device to be used and in the washing and drying process which is more complex with tubular films than with flat films.
Processes for the manufacture of cellulosic tubular films are, moreover, well known from U.S. Pat. No. 5,277,857 and EP-A-0 662 283. According to these known processes a cellulose solution is formed to a tube via an extrusion nozzle with a ring-shaped extrusion gap, said tube being drawn over a cylindrical mandrel and introduced to a precipitation bath. In order that the extruded tube does not stick to the surface of the mandrel its surface is covered with a water film so that the inner side of the tube coagulates and slides over the cylindrical mandrel. According to EP-A-0 662 283 the tubular film is extended after washing by blowing in a gas.
DE-C-44 21482 describes a blowing process for the manufacture of oriented cellulosic films whereby the cellulose solution is extruded via a film blowing nozzle and an air gap downwards into a precipitation bath. It is mentioned that stretching can be performed transverse to the transport direction of the blown film via the gas pressure in the inside of the blown film and that the relation of mechanical longitudinal and transverse properties can be set.
A process and a device for the manufacture of cellulosic films, particularly of tubular films, is also known from WO-A-95/07811 of the applicant. In this respect the dissolved cellulose is cooled before it is brought into the precipitation bath by subjecting the heated solution to a stream of gas immediately after extruding.
From WO-A-97/24215 there is known a process for the production of an oriented cellulosic film in which a cellulosic solution is applied to a stretchable surface to which the solution sticks, the solution is then stretched by stretching the extendable surface and finally it is precipitated.
From EP-B-0 494 851 of the patent applicant there is known a process for the production of a cellulosic flat film in which a cellulosic solution is passed through a nozzle or gap, it is then led through an air gap and then coagulated in a precipitation bath and the coagulated flat film is stretched in the longitudinal direction.
With cellulosic membranes particularly in the form of flat membranes, i.e. membranes from a flat film, the permeability of the membranes is an important property. To solve certain separation tasks it is important to select membranes with the optimum permeability, pore size and pore structure for the respective separation task.
Dialysis membranes made of regenerated cellulose in the form of flat films, tubular films or hollow threads have been known for some time whereby the regeneration of the cellulose can take place by means of the cuoxam-process, the viscose process or by means of the hydrolysis of cellulose acetate. Depending upon the process used and the process conditions one obtains membranes with different dialysis properties.
U.S. Pat. No. 4,354,938 describes for example a process for the production of dialysis membranes according to the viscose process, in which a tubular moulded membrane is stretched in the transverse direction by between 40 and 120% by blowing up with air before drying, which leads to a membrane with a regular orientation in the longitudinal and transverse direction. When transferring the dried membranes in the wet state the membranes thus produced undergo a shrinkage in the longitudinal and transverse direction of 0.5-10%. The ultrafiltration values lie in the range of between 2.5 ml/m2.h.mm Hg and 5.2 ml/m2.h.mm Hg at a wet thickness of 184 xcexcm to 45 xcexcm.
In xe2x80x9cmembranes and membrane processesxe2x80x9d by E. Staude, 1992, VCH Verlagsges.m.b.H. on page 19 it is described that the biaxial stretching of finished cellophase membranes leads to the enlargement of the pores, monoaxial stretching on the other hand leads to a reduction in the effective pore diameter.
However, the viscose process only offers limited possibilities to set membrane properties in a well aimed manner. Moreover, the recovery of the chemicals which accumulate in this process such as sodium sulphate and carbon bisulphide etc. is very expensive.
The present invention has as an object to provide a process for the production of cellulosic flat films with improved mechanical properties. Moreover, it is the object of the invention to provide a process for the production of cellulosic membranes in the form of flat membranes by means of which membranes with a permeability which is optimised for the individual separation task to be accomplished can be achieved.
This object is achieved by a process for the production of cellulosic flat films and cellulosic membranes in the form of flat membranes whereby a solution of cellulose in an aqueous tertiary amine oxide is moulded in the form of a film using an extrusion nozzle which has an oblong extrusion gap, said solution being led through an air gap into a precipitation bath whereby the cellulosic flat film is formed in the precipitation bath, and in which according to the invention the cellulosic flat film is stretched in the transverse direction after entering the precipitation bath.
The transverse stretching can thereby take place in the precipitation bath or at a later time. Under transverse stretching is understood a stretching in the direction of the width of the cellulosic flat film.
It is known that cellulose films made according to the viscose process can hardly be shaped at all after regenerating and can only be stretched in the transverse direction to a limited extent.
Surprisingly, in accordance with the invention it was shown that in the case of films manufactured according to the amine oxide process according to the generic term of claim 1, transverse stretching is also possible after the precipitation of the film-like moulded solution of cellulose. In this way cellulosic flat films are obtained with improved mechanical properties. The costly blowing up of a tubular moulded cellulosic solution in the air gap is thereby not necessary.
The transverse stretching of the films can be performed according to well known methods such as for example those used with thermoplastic films (as is for example described in the Handbook of Plastics Extrusion II, extrusion plants, Hanser-Verlag, 1986, 261-269), for example by conveyor belts or by clamping devices which are attached to endless belts respectively chains by the fact that the belts are led in divergent directions.
Preferably the cellulose solution is extruded using an extrusion die, which has an extrusion gap with a length of at least 40 cm. The cellulose solution can, however, also be extruded from an extrusion gap with a length of less than 40 cm which results in films with a lower width. The extrusion die preferably includes an extrusion section. The cellulose solution flows out from the extrusion section into the extrusion gap. The length of the extrusion section is preferably greater than 1 mm, most preferably from about 5 mm to about 20 mm.
According to one preferred embodiment of the process in accordance with the invention, the cellulosic flat film is stretched in the longitudinal direction in the air gap, preferably in a range of 0.2 to 5 times.
One further advantageous embodiment of the process according to the invention is characterized in that the cellulose flat film is first washed after precipitation and stretched after washing.
Surprisingly it has been shown that cellulosic flat films made using the process according to the invention can be stretched in the transverse direction in a washed state by up to 3.5 times their original width.
In accordance with one further preferred embodiment of the process in accordance with the invention the cellulosic flat film is first of all washed after precipitation and dried and after that the dry cellulosic flat film is moistened and stretched preferably by spraying with water. Surprisingly it was shown that cellulosic flat films treated in this way can be stretched by up to 3.5 times their original width in the transverse direction.
The process according to the invention has the advantage that by stretching the cellulosic flat film up to 3.5 times the original width in the transverse direction the mechanical properties of the film can be set in a wide range in the longitudinal and transverse direction.
Preferably N-Methylmorpholine-N-oxide (NMMO) is used as the tertiary amine oxide.
The advantages described above of the process in accordance with the invention apply in particular to a production in accordance with the invention of cellulosic membranes in the form of flat membranes.
In this respect the permeability of the membrane and thus also its ultrafiltration rate (UFR) can be influenced in particular by the selection of the speed at which the film-like moulded solution is drawn off in the air gap. In this respect it is shown that a lower draw-off speed increases the permeability and thus also the ultrafiltration rate of the membrane. By contrast the transverse stretching of the film after entering the precipitation bath increases the permeability of the membrane. Thus basic membrane properties can be controlled by the selection of draw-off speed and the transverse stretching of the film.
The invention also relates to the use of a cellulosic flat film made by the process in accordance with the invention as a package material, in particular for foodstuff, as a material for waste and carrier bags, as film for agricultural applications, as a film for diapers, as a substrate for compounds, as an office film, as a household film or as a membrane to separate substance mixtures.
With the following examples the invention is described in greater detail. The cellulose solutions used were manufactured in accordance with the process described in EP-A-0 356 419. In all the examples the films were washed following precipitation of the cellulose and treated with glycerine (glycerine content of the dried film about 15 wt. %) and finally dried on a tenter frame, in which the films were fixed in the longitudinal and transverse direction. The properties listed in the examples were determined with the dried films, whereby the tenacity (longitudinal and transverse) and the longitudinal and transverse elongation were determined according to DIN 53457.
The ultrafiltration rate given in the examples is defined as the per time unit of the volume of the permeate passing through the membrane wall relative to the membrane area and test pressure.   UFR  =            V              t        ·        A        ·        p              ⁢          ml              h        ·                  m          2                ·                  mm          ⁢          Hg                    
V=volume of liquid (permeate) [ml]
t=time [h]
A=membrane area [m2]
p=test pressure [mm Hg]The values for diffuse permabilities are determined by the increase of the straight line arrived at by plotting ln(ct/co) against time.       ln    ⁢          xe2x80x83        ⁢                  C        t                    C        o              =            -              A        V              ·          P              diff        I            
co=starting concentration
ct=concentration at time t
A=membrane area [cm2]
V=dialysis volume [cm3]
Pdiff=diffuse permeability [cm/min]
t=time [min]
To directly compare the different membranes all the permeabilities were converted to a wet thickness of 75 xcexcm, i.e. the corresponding times for reaching the balanced state were standardized to this thickness. For example if with a membrane with a thickness of 200 xcexcm, the balanced state of NaCl-dialysis was reached after 100 hours, then this corresponds to a time of 100xc3x9775/200=37.5 hours for a membrane with a thickness of 75 xcexcm.